Primary vs Secondary Growth-4

Residual Procambium between Primary Phloem & Primary Xylem Remains Mitotically Active & becomes Vascular Cambium.

Vascular Cambium divides Periclinally & produces Radial Files of Cells.

Cells displaced towards the Inside become Secondary Xylem

Cells displaced towards the Outside become Secondary Phloem.

Cells of the Interfascicular Zone** are Induced to Divide to become the Interfascicular Vascular Cambium.

Vascular Cambia Merge to form a Continuous Ring.

Further Cambial Activity yields Secondary Xylem that becomes Wood

As Wood Volume Increases, pressure develops this crushes the  oldest Phloem.

New, Active Phloem is continuously produced by the Vascular Cambium.

Secondary Vascular Tissues are Important for many reasons. These include:

Secondary Xylem or Wood is Essential for Increased Height which can lead to the Ecological Dominance of the aerial Environment.This is due to the dual properties of wood which are its innate strength and its ability to transport water over long distances. It also resists decay and allows the plant to accumulate mass over time. Consequently it does not need to start from ground zero each year, like herbaceous plants.

Secondary growth in Roots produces a root system that is capable of anchoring large trees & for the development of enormous root systems that absorb minerals and water from the soil.

The increased volume of Secondary Phloem results in a greater ability to transport sugars.

Secondary Growth also occurs in Roots.

The only difference is that Roots do not have a Pith because Primary Xylem is present in the center of the root.

**Most Americans probably tuned into the recent Impeachment Trial in the US Senate. If you have ever been to Washington DC you will see how the early architects and artists for governmental buildings were impressed with Greco-Roman architecture and symbols. On each side of the seat occupied by the presiding officer of the Senate are two Fasces. Not two Faces, although this would be very appropriate!  The Romans had many symbols of Power. One of them was a bundle of sticks lashed together in a cylinder with a long axe in the center. This is a Fasces! Early Botanists noted that the vascular tissue in stems appeared in discrete bundles which they called Fascicles!!! Fasces -> Fascicles.  An area of Ground Tissue between the Fascicles was called Interfascicular! Hence, the cambial layer which develops in the Interfascicular Region is called Interfascicular!

Diagram of a Stem at the End of Primary Growth.

Coleus Stem at the end of Primary growth: Note the similarity to the Diagram Above.

Same as above but viewed with Polarized Light

Same as above after some Secondary Growth: Secondary Xylem is prominent.

Diagram showing Tissues following a small amount of Secondary Growth

Diagrams comparing Primary & Secondary growth in a Model Stem




Woody Stem after one year's growth

Woody Stem after two year's growth

Woody Stem after many years of growth

Root with Primary Vascular Tissues(Haplostele)

Diagram of a Root at the end of Primary Growth


Diagram of a Root after some Secondary Growth.